Conventionally, there is used a technology to record a large volume of information on an optical information recording medium, e.g., an optical disc such as a write once type optical disc (CD-R). And, the technology for recording information at 2–6X speed or a faster recording speed is coming into general use in recent years.
When recording information on an optical disc, based on a digital signal of digitized information to be recorded, pulse-shaped laser light is irradiated from an optical pickup to the rotating optical disc to form pits.
For example, to form pits on the optical disc by using the digital signal of the digitized information to be recorded, the intensity of laser light irradiated to the optical disc during a high-level period of the digital signal is controlled to be high to change the state of a recording layer by the laser light energy so to form the pits. And, the laser light having a low light intensity needed for tracking is irradiated during a low-level period.
The optical disc might have an inclined information recording surface due to warp or deformation of the disc. Mostly, the inclination of the information recording surface is not uniform and becomes large gradually from the inner circumference toward the outer circumference of the optical disc. Inclination angle θ of the information recording surface to a plane surface orthogonal to the rotation axis of the optical disc is generally called the tilt angle.
When the information recording surface is not inclined, the laser light is irradiated substantially vertically from the optical pickup to the information recording surface to form the pits.
But, when the direction that the laser light is emitted is fixed and if the information recording surface is inclined, a spot of the laser light is distorted, so that a light amount irradiated per unit area of the information recording surface decreases. Therefore, the pit size becomes small, and jitter becomes worse when the information is reproduced.
Therefore, there is adopted tilt servo control which detects the inclination (tilt angle) of the information recording surface of the optical disc and tilts the optical pickup according to the tilt angle so to control the direction to emit the laser light to be perpendicular to the information recording surface of the optical disc.
In addition to the above tilt servo control, servo control such as focusing control of the laser light spot and tracking control so to make the laser light spot follow grooves is performed to form optimum pits.
However, the mechanism to incline the optical pickup according to the tilt angle θ is complex, and the device costs a great deal.
A method for appropriate recording (appropriate β) without using a tilt correction mechanism is disclosed in “Power calibration method for optical disc recording/reproducing device” (Japanese Patent Application Laid-Open Publication No. 8-147701). It is suggested to make the optimization of recording laser light intensity on the outer circumference (hereinafter called OPC: Optimum Power Control), but OPC takes time.
The aforesaid β means asymmetry, which is expressed by the following expression (A) or (B).β=(A1+A2)/(A1−A2)  (A)β=[(I14H+I14L)−(I3H+I3L)]/[2(I14H−I14L)]  (B)where, A1 means a top level value of a pulse having length 11T of CD-R and A2 means a bottom level value of a pulse having length 11T.
And, I14H means a top level value of a pulse having length 14T of DVD-R, I14L means a bottom level value of the pulse having length 14T, I3H means a top level value of a pulse having length 3T, and I3L means a bottom level value of the pulse having length 3T.